Digital halftoning is a technique employing digital image processing to produce a halftone output image from a continuous-tone input image. In the digital color image halftoning technique, a continuous-tone color image is sampled, for example with a scanner, the samples are digitized and stored in a computer. The digitized samples or pixels consist of three binary representations of red, green, and blue scanned color values typically ranging from 0 to 255 for each color. To recreate this image on a printer that is capable of printing dots of only one brightness for each output color, it is necessary to create the sensation of multi-level colors by suitably distributing the printed dots in the output image. For three color printers, e.g., those that employ cyan, magenta, and yellow toners or inks, this is a mapping at each pixel from three multi-level digital input values to one of eight, i.e., 2.sup.3, possible output values. Likewise for four color printers that employ black toner or ink there are sixteen or 2.sup.4 possible output values. Any binary combination of the three or four colors can be produced at each output pixel. One prior art method of color digital halftoning is to use clustered-dot halftone screens, one for each color, to independently threshold the input color values to produce the output binary combinations. This is typically proceeded by some sort of color processing to convert the input red, green, and blue colors into continuous-tone output cyan, magenta, yellow, and black colors for screening. There are many types of screens including adaptive screens that vary with the input signal, and many methods for introducing offsets or rotations among the screens to reduce low frequency color harmonics or Moire patterns in uniform image areas. At the high dot densities, e.g., 1800 dots/inch, of high quality graphics the fundamental screen patterns are not visible to the unaided eye. For copiers and medium-to-low quality printers, e.g., 300-500 dots/inch, however, these patterns are visible, and increasing the screen frequency to reduce their visibility reduces the number of levels and introduces false gradations or contours in the output image.
Another prior art method is to use independent error diffusion on each color. In this method, each color is processed as a separate continuous-tone signal using the techniques of error diffusion to propagate binarization error to subsequent pixels producing the perception of multiple output pixels. Although superior to screening in terms of the visibility of binary noise, this technique cannot produce optimal output color quality because it does not use the input color information interactively at each pixel, nor does it employ knowledge of the spatial response of the human visual system to color signals.
It is the object of the present invention to provide an improved color error diffusion digital halftoning technique that avoids the problems noted above.